1. Field of the Invention
The present invention relates to a muffler with a built-in heat exchanger.
2. Description of the Related Art
Internal combustion engines are equipped with an exhaust system for discharging spent gas into the atmosphere, while at the same time reducing noise and the contaminating substances in the gas. Modern exhaust systems include at least a catalytic converter followed by at least a muffler in which a labyrinth is formed defining a path along which the exhaust gas flows from the inlet to the outlet.
To improve overall energy efficiency, it has recently been proposed to also make use of part of the heat in the exhaust system, which is normally dispersed into the atmosphere, for purposes such as heating the engine cooling fluid when starting the engine, to reach running temperature faster. One proposed solution is to employ a liquid-gas heat exchanger installed along the path of the exhaust gas so the exhaust gas flows through it. To selectively feed the exhaust gas through the heat exchanger in specific conditions only (e.g. when the engine is cold, and the heat in the exhaust gas is used to heat the engine cooling fluid faster), it has been proposed to employ a bypass conduit controlled by a bypass valve and enabling the exhaust gas to bypass the heat exchanger. One example of an exhaust system heat exchanger of this type is described in Patent Application WO2005024193A1.
To reduce cost and size, it has been proposed to build the heat exchanger into the muffler, as described in Patent Applications JP2004245127A1 and WO2008117580A1. These known solutions, however, have several drawbacks, foremost of which is the complex design of the bypass valve, in which the shutter is integral with a very long shaft that has to come out of the main body of the muffler through a gastight hole; and, because of its length, the shaft is subject to severe thermal expansion by the hot (at least 250-300° C.) exhaust gas flowing through the muffler. Moreover, when the bypass valve in these known solutions is open (which is most of the time), the noise-deadening efficiency of the muffler is poor compared with a similar standard muffler with no heat exchanger. Both these known solutions are complicated and expensive to produce.
Patent Application JP2007032561A1 describes a muffler with a built-in heat exchanger, in which a valve directs the exhaust gas selectively along a heat exchange path or along a bypass path parallel to the heat exchange path; and thermal insulation is provided between the bypass path and heat exchange path to reduce heat exchange between the two.